$ A = \left[\begin{array}{rr}0 & -1 \\ 2 & 1\end{array}\right]$ $ w = \left[\begin{array}{r}2 \\ -1\end{array}\right]$ What is $ A w$ ?
Answer: Because $ A$ has dimensions $(2\times2)$ and $ w$ has dimensions $(2\times1)$ , the answer matrix will have dimensions $(2\times1)$ $ A w = \left[\begin{array}{rr}{0} & {-1} \\ {2} & {1}\end{array}\right] \left[\begin{array}{r}{2} \\ {-1}\end{array}\right] = \left[\begin{array}{r}? \\ ?\end{array}\right] $ To find the element at any row $i$ , column $j$ of the answer matrix, multiply the elements in row $i$ of the first matrix, $ A$ , with the corresponding elements in column $j$ of the second matrix, $ w$ , and add the products together. So, to find the element at row 1, column 1 of the answer matrix, multiply the first element in ${\text{row }1}$ of $ A$ with the first element in ${\text{column }1}$ of $ w$ , then multiply the second element in ${\text{row }1}$ of $ A$ with the second element in ${\text{column }1}$ of $ w$ , and so on. Add the products together. $ \left[\begin{array}{r}{0}\cdot{2}+{-1}\cdot{-1} \\ ?\end{array}\right] $ Likewise, to find the element at row 2, column 1 of the answer matrix, multiply the elements in ${\text{row }2}$ of $ A$ with the corresponding elements in ${\text{column }1}$ of $ w$ and add the products together. $ \left[\begin{array}{r}{0}\cdot{2}+{-1}\cdot{-1} \\ {2}\cdot{2}+{1}\cdot{-1}\end{array}\right] $ Fill out the rest: $ \left[\begin{array}{r}{0}\cdot{2}+{-1}\cdot{-1} \\ {2}\cdot{2}+{1}\cdot{-1}\end{array}\right] $ After simplifying, we end up with: $ \left[\begin{array}{r}1 \\ 3\end{array}\right] $